docs(nwsync): verify is what tells you which keys to purge

Running the repair for sow-tools#88 disproved the advice #90 had just
landed. "Purge the zone, then believe verify" assumed the stale set was
unknowable. It is not: verify reads the edge, so a run straight after a
repair names every key the edge is still serving stale — a survey, not a
verdict. Purge those, re-run, and the second run is the verdict.

The measured numbers are the argument. The repair rewrote 2,603 blobs at
the origin; 8 were stale at the edge, all of them ones a failed player
sync had pulled ninety minutes earlier. The edge only caches what someone
fetched, so purging the whole zone would have cooled 69,169 objects to
fix 8.

Refs #88, #89, #75.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.md

@@ -19,7 +19,7 @@ Crucible is how the artifact repos turn source into artifacts.
 | `crucible-depot`   | `crucible depot`   | content-addressed depot blob verify/move        |
 | `crucible-hak`     | `crucible hak`     | ERF/HAK pack/unpack + hak manifests             |
 | `crucible-module`  | `crucible module`  | build/extract/validate/compare the `.mod`       |
-| `crucible-nwsync`  | `crucible nwsync`  | NWSync blob emit + manifest assemble            |
+| `crucible-nwsync`  | `crucible nwsync`  | NWSync blob emit + manifest assemble + verify   |
 | `crucible-topdata` | `crucible topdata` | compile 2da/tlk topdata + packages              |
 | `crucible-wiki`    | `crucible wiki`    | render + deploy mechanical wiki pages           |
 

docs/command-surface.md

@@ -36,6 +36,7 @@ aliases.
 | `depot` | `pull` | Incremental verified pull of every referenced blob. |
 | `nwsync` | `emit` | Explode one artifact into NWSync blobs plus its own NSYM manifest. |
 | `nwsync` | `assemble` | Merge per-artifact NSYM manifests into one merged manifest. |
+| `nwsync` | `verify` | Decompress and hash a published manifest's blobs through the pull zone. |
 
 `depot status` and `depot get` pick their backend either with `--out DIR`, a
 depot tree on disk, or with `--target bunny|cdn`, a remote backend. The two
@@ -48,8 +49,9 @@ beside the artifact itself with the extension replaced, so `emit` and
 `assemble` agree on where it is without being told.
 
 ```
-nwsync emit     [--as NAME] [--out DIR] [--jobs N] <artifact-key> <file>
+nwsync emit     [--as NAME] [--out DIR] [--jobs N] [--verify] <artifact-key> <file>
 nwsync assemble --group-id N [--tlk-key KEY] [--out DIR] <artifact-key>...
+nwsync verify   [--sample N] [--base URL] [--jobs N] <manifest-sha1>
 ```
 
 `emit` is latency-bound, not CPU-bound: every blob costs an existence probe
@@ -73,6 +75,45 @@ resolves it. `--tlk-key` has its own slot because the TLK shadows nothing.
 `--group-id` is per channel — 1 is current, 2 is testing, and 0 leaves the field
 out of the sidecar.
 
+`nwsync verify` is the only check on a published blob upstream of a player's
+client. It reads the **pull zone**, not the storage API, and needs no
+credential: what matters is the bytes a client is served, edge behaviour
+included. Every blob is decompressed and hashed, and the zstd frame is asserted
+to declare its content size. Neither half is optional — a `Content-Length` check
+passes a byte-correct-looking object whose contents are short, and a round-trip
+check alone passes a frame the game client cannot decode but Go's decoder can.
+Failures are reported per blob as missing, malformed framing, size mismatch or
+hash mismatch, and the exit code is 1.
+
+A full sweep of the live manifest is roughly 69,000 blobs and 15 GB, so
+`--sample N` exists to make verifying routine; the default is a full sweep.
+`--base URL` (or `NWSYNC_PULL_BASE`) overrides the public host.
+
+`emit --verify` applies the same check where `emit` would otherwise skip. `emit`
+normally reads a blob's presence as proof of its contents, decided by a 1-byte
+range GET, so an object written truncated — or written by an emitter since found
+broken — is skipped by every later run forever and no backfill repairs it. With
+`--verify` the stored copy is read back, unwrapped, hashed against its own name,
+and replaced when it does not match. It costs a full GET per existing blob, so
+it is a repair pass, not the default.
+
+**After a repair, `verify` is what tells you which keys to purge.** A repair is
+the one thing that makes a key serve different bytes than it did before, and the
+edge caches these objects for 30 days precisely because that normally cannot
+happen. The two commands look at different copies on purpose: `emit --verify`
+repairs the **origin**, `verify` reads the **edge**. So a `verify` run straight
+after a repair is not a verdict — it is a survey, and every blob it still calls
+bad is one the edge is serving stale. Purge exactly those, then re-run it; only
+that second run is the verdict.
+
+Purging the keys `verify` names beats purging the zone, because the edge only
+ever cached what somebody actually fetched: the 2026-08-01 repair rewrote 2,603
+blobs at the origin and left 8 stale at the edge. The purge belongs in the
+repair procedure rather than in `emit`, which reports how many blobs it wrote
+and never which ones — so it could not target one even with a CDN credential,
+which it deliberately does not hold (#89; the procedure itself is in
+sow-platform's NWSync runbook).
+
 `emit` uploads blobs first and the index last, so the presence of an index is
 the publication marker: an artifact whose emit died halfway leaves real blobs in
 the zone and no index. Blob names are content hashes, so re-running skips
@@ -108,6 +149,17 @@ saving on hak content: a 250 MB hak emitted 2296 blobs totalling 59 MB on disk
 against 249 MB of resources, as recorded in the sidecar's `on_disk_bytes` and
 `total_bytes`.
 
+The zstd frame always declares its `Frame_Content_Size`. The game client sizes
+its output buffer from that field and cannot decode a frame without one, but the
+Go encoder omits it below 256 bytes, so `emit` re-headers those frames into the
+shape reference libzstd emits: `Single_Segment_flag` set, `Window_Descriptor`
+dropped, and a one-byte content size in its place. `zstd -l <frame>` must print a
+decompressed size; a blank column there is the fault, and it is invisible to any
+check that only decompresses, because both `zstd -dc` and Go's decoder stream
+such a frame happily. This is what the sidecar's `emitter_version` counts:
+version 1 omitted the field and no client could sync past such a blob, version 2
+declares it. `assemble` refuses to merge indexes that disagree.
+
 ## Hidden compatibility aliases
 
 Existing scripts may continue using these names indefinitely. They are accepted

internal/app/app.go

@@ -385,7 +385,7 @@ func refreshBuildModuleManifest(ctx context, p *project.Project, progress func(s
 	}
 
 	progress("Refreshing hak list from the latest published sow-assets manifest...")
-	if err := runProjectScript(ctx, p, []string{"scripts", "fetch-hak-manifest"}, manifestPath); err != nil {
+	if err := runProjectScript(ctx, p, []string{"scripts", "fetch-upstream-manifests"}, manifestPath); err != nil {
 		return "", "", err
 	}
 	if _, err := pipeline.ApplyHAKManifest(p, manifestPath); err != nil {

internal/dispatch/dispatch.go

@@ -108,10 +108,11 @@ var Registry = []Builder{
 	{
 		Name:    "nwsync",
 		Bin:     "crucible-nwsync",
-		Summary: "publish NWSync blobs and manifests (emit/assemble)",
+		Summary: "publish NWSync blobs and manifests (emit/assemble/verify)",
 		Commands: []Command{
 			{Name: "emit", Summary: "explode one artifact into blobs plus its own NSYM manifest", Usage: "crucible nwsync emit <artifact> --out DIR"},
 			{Name: "assemble", Summary: "merge per-artifact NSYM manifests into one", Usage: "crucible nwsync assemble --order NAMES --entries DIR --out DIR [--group-id N]"},
+			{Name: "verify", Summary: "read a published manifest's blobs back through the pull zone and hash them", Usage: "crucible nwsync verify <manifest-sha1> [--sample N]"},
 		},
 		Wired: true,
 	},

internal/dispatch/dispatch_test.go

@@ -160,7 +160,7 @@ func TestCanonicalCommandSurface(t *testing.T) {
 		"module":  {"build", "extract", "validate", "compare", "manifest"},
 		"topdata": {"validate", "build", "package", "compare", "convert"},
 		"wiki":    {"build", "deploy"},
-		"nwsync":  {"emit", "assemble"},
+		"nwsync":  {"emit", "assemble", "verify"},
 	}
 	for _, builder := range Registry {
 		got := builder.subcommands()

internal/nwsync/compressedbuf.go

@@ -3,6 +3,7 @@ package nwsync
 import (
 	"bytes"
 	"encoding/binary"
+	"encoding/hex"
 	"fmt"
 
 	"github.com/klauspost/compress/zstd"
@@ -31,6 +32,19 @@ var (
 	blobDecoder, _ = zstd.NewReader(nil, zstd.WithDecoderConcurrency(1))
 )
 
+// zstd frame header bits we care about. A frame starts with the magic, then a
+// one-byte Frame_Header_Descriptor: bits 7-6 size the Frame_Content_Size field,
+// bit 5 is Single_Segment_flag, bits 1-0 size the Dictionary_ID field.
+const (
+	zstdFrameMagic      = "\x28\xb5\x2f\xfd"
+	frameSingleSegment  = 1 << 5
+	frameDictionaryMask = 0x03
+	// oneByteContentSizeCeiling is the size above which a Frame_Content_Size no
+	// longer fits in one byte. Below it the field's size flag is 0, which is
+	// what lets klauspost/compress leave the field out entirely.
+	oneByteContentSizeCeiling = 256
+)
+
 // compressBlob wraps data in NWCompressedBuffer framing.
 func compressBlob(data []byte) []byte {
 	var out bytes.Buffer
@@ -38,10 +52,95 @@ func compressBlob(data []byte) []byte {
 	for _, field := range header {
 		_ = binary.Write(&out, binary.LittleEndian, field)
 	}
-	out.Write(blobEncoder.EncodeAll(data, nil))
+	frame := declareFrameContentSize(blobEncoder.EncodeAll(data, nil), len(data))
+	// Fail closed rather than publish a blob no client can decode. An encoder
+	// upgrade that finds a new way to omit the field would otherwise reproduce
+	// #86 in silence, and a blob is skipped by every later emit once written.
+	if !frameDeclaresContentSize(frame) {
+		panic(fmt.Sprintf("nwsync: refusing to emit a %d-byte blob whose zstd frame declares no content size (descriptor %#x)",
+			len(data), frame[4]))
+	}
+	out.Write(frame)
 	return out.Bytes()
 }
 
+// inspectBlob unwraps a stored blob the way the game client reads it, and is the
+// only reader that should be trusted to judge a published blob.
+//
+// It asserts the frame property on top of the round trip. Go's decoder — like
+// the zstd CLI — streams a frame that declares no content size, so a check that
+// only decompresses and hashes is a *more* capable decoder than the client's: it
+// certifies exactly the blobs the client rejects, which is how #86 reached
+// production and survived an audit.
+func inspectBlob(blob []byte) ([]byte, error) {
+	data, err := decompressBlob(blob)
+	if err != nil {
+		return nil, fmt.Errorf("malformed framing: %w", err)
+	}
+	if len(blob) > blobHeaderBytes && !frameDeclaresContentSize(blob[blobHeaderBytes:]) {
+		return nil, fmt.Errorf("malformed framing: the zstd frame declares no content size, which the game client cannot decode")
+	}
+	return data, nil
+}
+
+// blobMatchesName holds a stored blob to its own file name: a blob is named
+// after the sha1 of its uncompressed bytes, so the name is a complete statement
+// about the contents and nothing else is needed to check it.
+func blobMatchesName(blob []byte, sha1Hex string) error {
+	data, err := inspectBlob(blob)
+	if err != nil {
+		return err
+	}
+	if got := hex.EncodeToString(sha1Sum(data)); got != sha1Hex {
+		return fmt.Errorf("blob %s holds the contents of %s", sha1Hex, got)
+	}
+	return nil
+}
+
+// frameDeclaresContentSize reports whether a zstd frame states how many bytes it
+// decompresses to. A frame with a zero-sized Frame_Content_Size field declares
+// one only when Single_Segment_flag is set; otherwise the size is unknown.
+func frameDeclaresContentSize(frame []byte) bool {
+	if len(frame) < 5 || string(frame[:4]) != zstdFrameMagic {
+		return false
+	}
+	descriptor := frame[4]
+	return descriptor>>6 != 0 || descriptor&frameSingleSegment != 0
+}
+
+// declareFrameContentSize rewrites a frame that does not declare its
+// Frame_Content_Size so that it does, and returns any other frame unchanged.
+//
+// klauspost/compress omits the field for inputs under 256 bytes, which the spec
+// permits. Reference libzstd never does, so the NWN client — which sizes its
+// output buffer from ZSTD_getFrameContentSize and has therefore never met a
+// frame without one — rejects the blob outright with an empty "potential
+// compression error" (#86). No encoder option changes this, so the frame is
+// re-headered here.
+//
+// The result is the shape libzstd itself emits for the same input: setting
+// Single_Segment_flag drops the Window_Descriptor byte, and the freed byte pays
+// for a one-byte Frame_Content_Size. Window_Size then equals the content size,
+// which is sound because the content is under 256 bytes and every match in it
+// therefore falls inside that window. Same length in, same length out.
+func declareFrameContentSize(frame []byte, size int) []byte {
+	if size <= 0 || size >= oneByteContentSizeCeiling || len(frame) < 6 || string(frame[:4]) != zstdFrameMagic {
+		return frame
+	}
+	descriptor := frame[4]
+	// Rewrite only the exact shape a small input produces: no declared size, no
+	// single segment, no dictionary. Anything else either declares a size
+	// already or is not a frame this reinterpretation is safe on.
+	if descriptor>>6 != 0 || descriptor&frameSingleSegment != 0 || descriptor&frameDictionaryMask != 0 {
+		return frame
+	}
+	reframed := make([]byte, len(frame))
+	copy(reframed, frame)
+	reframed[4] = descriptor | frameSingleSegment
+	reframed[5] = byte(size) // replaces Window_Descriptor
+	return reframed
+}
+
 // decompressBlob unwraps NWCompressedBuffer framing. It exists so a blob this
 // package wrote — or one upstream wrote — can be compared by its uncompressed
 // bytes, which is the only comparison that is meaningful across zstd

internal/nwsync/emit.go

@@ -33,7 +33,9 @@ var skippedTypes = resTypes("nss", "ndb", "gic")
 // manifest quietly disagrees with. Bump it only when emitted bytes change — it
 // is deliberately not the build revision, which would invalidate every
 // published index on every unrelated commit.
-const emitterVersion = "1"
+// Version 2 declares Frame_Content_Size on every blob (#86); version 1 omitted
+// it below 256 bytes and no client could sync past such a blob.
+const emitterVersion = "2"
 
 // serverTypes are loaded only server-side; a manifest holding nothing else
 // has no client contents. Mirrors upstream's GlobalResTypeServerList, whose
@@ -78,6 +80,7 @@ type EmitOptions struct {
 	As           string // name override, for a TLK whose filename is not its published name
 	OutDir       string // write locally instead of uploading — the conformance path
 	Jobs         int    // resources in flight at once; 0 means defaultEmitJobs
+	Verify       bool   // hash what would be skipped instead of trusting presence
 	Sink         sink   // test seam; nil means OutDir or the zone
 }
 
@@ -129,7 +132,7 @@ func Emit(options EmitOptions) (EmitResult, error) {
 	if jobs < 1 {
 		jobs = defaultEmitJobs
 	}
-	entries, blobs, onDiskBytes, err := emitResources(artifact, index, target, jobs)
+	entries, blobs, onDiskBytes, err := emitResources(artifact, index, target, jobs, options.Verify)
 	if err != nil {
 		return EmitResult{}, err
 	}
@@ -199,7 +202,7 @@ func readArtifactIndex(path string, artifact io.ReaderAt, size int64, name strin
 //
 // The returned entries are in artifact order whatever order the workers finish
 // in, because a manifest's bytes are promised deterministic by emitterVersion.
-func emitResources(artifact io.ReaderAt, index []erf.IndexEntry, target sink, jobs int) ([]Entry, int, int64, error) {
+func emitResources(artifact io.ReaderAt, index []erf.IndexEntry, target sink, jobs int, verify bool) ([]Entry, int, int64, error) {
 	// A resref appearing twice inside one artifact resolves to the last one,
 	// the way resman lets the last container added win.
 	order := make([]Identity, 0, len(index))
@@ -271,7 +274,7 @@ func emitResources(artifact io.ReaderAt, index []erf.IndexEntry, target sink, jo
 		if duplicate {
 			return
 		}
-		written, err := target.putBlob(fmt.Sprintf("%x", sum), func() []byte { return compressBlob(payload) })
+		written, err := target.putBlob(fmt.Sprintf("%x", sum), verify, func() []byte { return compressBlob(payload) })
 		mu.Lock()
 		defer mu.Unlock()
 		if err != nil {

internal/nwsync/manifest.go

@@ -7,6 +7,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
+	"path"
 	"path/filepath"
 	"sort"
 	"strings"
@@ -194,7 +195,14 @@ func marshalSidecar(sidecar Sidecar) ([]byte, error) {
 	return append(body, '\r', '\n'), nil
 }
 
-// blobPath is the data store path for a blob, hash tree depth 2.
+// blobKey is where a blob lives in a zone, hash tree depth 2. emit writes it,
+// verify reads it and the game client requests it, so the rule lives here and
+// nowhere else.
+func blobKey(sha1Hex string) string {
+	return path.Join("data", "sha1", sha1Hex[0:2], sha1Hex[2:4], sha1Hex)
+}
+
+// blobPath is the same location inside a local repository tree.
 func blobPath(root, sha1Hex string) string {
-	return filepath.Join(root, "data", "sha1", sha1Hex[0:2], sha1Hex[2:4], sha1Hex)
+	return filepath.Join(root, filepath.FromSlash(blobKey(sha1Hex)))
 }

internal/nwsync/nwsync_test.go

@@ -458,6 +458,33 @@ func TestAssembleFailsClosedOnMissingIndex(t *testing.T) {
 	}
 }
 
+// Callers capture a script's stdout as a value: `dir="$(pack-haks.sh)"`. A
+// summary line on stdout gets glued onto that value, so both summaries belong
+// on stderr.
+func TestRunKeepsSummariesOffStdout(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "sow_top.hak")
+	writeHak(t, path, map[string][]byte{"appearance.2da": []byte("2da from sow_top")})
+	key := artifactKey(t, path)
+	out := filepath.Join(dir, "out")
+
+	for _, args := range [][]string{
+		{"emit", "--out", out, "--as", "sow_top.hak", key, path},
+		{"assemble", "--out", out, key},
+	} {
+		var stdout, stderr bytes.Buffer
+		if code := Run(args, &stdout, &stderr); code != exitOK {
+			t.Fatalf("Run(%v) exit=%d: %s", args, code, stderr.String())
+		}
+		if stdout.Len() != 0 {
+			t.Errorf("Run(%v) wrote to stdout: %q", args, stdout.String())
+		}
+		if stderr.Len() == 0 {
+			t.Errorf("Run(%v) reported no summary on stderr", args)
+		}
+	}
+}
+
 func TestRunUsageErrors(t *testing.T) {
 	cases := [][]string{
 		nil,
@@ -474,3 +501,31 @@ func TestRunUsageErrors(t *testing.T) {
 		}
 	}
 }
+
+// TestEveryBlobDeclaresItsFrameContentSize guards the fault that stopped every
+// client sync (#86): klauspost/compress omits Frame_Content_Size for inputs
+// under 256 bytes, and the game client cannot decode a frame without it. This
+// asserts a frame property, not a round trip — the zstd CLI and Go's decoder
+// both stream such a frame happily, so round-tripping cannot see the defect.
+func TestEveryBlobDeclaresItsFrameContentSize(t *testing.T) {
+	// 230 and 175 are real sizes from the manifest that failed to sync; 255/256
+	// straddle the encoder's threshold.
+	for _, size := range []int{1, 32, 175, 230, 255, 256, 257, 1024, 5000} {
+		payload := make([]byte, size)
+		for i := range payload {
+			payload[i] = byte('a' + i%26)
+		}
+		blob := compressBlob(payload)
+		if !frameDeclaresContentSize(blob[blobHeaderBytes:]) {
+			t.Errorf("blob of %d bytes declares no frame content size (descriptor %#x)",
+				size, blob[blobHeaderBytes+4])
+		}
+		got, err := decompressBlob(blob)
+		if err != nil {
+			t.Fatalf("decompress %d-byte blob: %v", size, err)
+		}
+		if !bytes.Equal(got, payload) {
+			t.Errorf("%d-byte blob did not round trip", size)
+		}
+	}
+}

internal/nwsync/run.go

@@ -12,12 +12,17 @@ import (
 	"flag"
 	"fmt"
 	"io"
+	"os"
 )
 
 const (
 	exitOK       = 0
 	exitUsage    = 64
 	exitInternal = 70
+	// exitDrift says the command worked and the zone is wrong, which is a
+	// different thing for CI to act on than the command failing. It matches
+	// depot's code for the same meaning.
+	exitDrift = 1
 )
 
 // Run executes an nwsync subcommand. args[0] is the subcommand (emit|assemble);
@@ -29,9 +34,11 @@ func Run(args []string, stdout, stderr io.Writer) int {
 	}
 	switch args[0] {
 	case "emit":
-		return runEmit(args[1:], stdout, stderr)
+		return runEmit(args[1:], stderr)
 	case "assemble":
-		return runAssemble(args[1:], stdout, stderr)
+		return runAssemble(args[1:], stderr)
+	case "verify":
+		return runVerify(args[1:], stdout, stderr, os.Getenv)
 	case "-h", "--help", "help":
 		printRunUsage(stdout)
 		return exitOK
@@ -44,17 +51,30 @@ func Run(args []string, stdout, stderr io.Writer) int {
 
 func printRunUsage(w io.Writer) {
 	fmt.Fprint(w, `usage:
-  nwsync emit     [--as NAME] [--out DIR] <artifact-key> <file>
+  nwsync emit     [--as NAME] [--out DIR] [--verify] <artifact-key> <file>
   nwsync assemble --group-id N [--tlk-key KEY] [--out DIR] <artifact-key>...
+  nwsync verify   [--sample N] [--base URL] <manifest-sha1>
 
 emit explodes one .hak/.erf or one loose file (the TLK) into NWSync blobs plus
 a NSYM index covering only that artifact, and uploads both. assemble merges
 those indexes into one manifest, reading no bulk data. Artifact keys are depot
 keys; an index lives beside its artifact, with the extension replaced.
 
+verify reads a published manifest and its blobs back through the public pull
+zone, with no credential, and decompresses and hashes every one. It is the only
+check on a published blob upstream of a player's client.
+
+--verify makes emit hash what it would otherwise skip. emit normally treats a
+blob's presence as proof of its contents, so without this an object written
+truncated, or written by an emitter since found broken, is skipped forever.
+--verify repairs the storage zone, while verify reads the edge in front of it.
+So a verify run right after a repair is a survey, not a verdict: it names the
+keys the edge still serves stale. Purge those, then run it again.
+
 --out DIR writes to a local repository tree instead of uploading, which is the
 conformance path against upstream nwn_nwsync_write. Without it, the zone comes
 from NWSYNC_STORAGE_ZONE, NWSYNC_STORAGE_PASSWORD and BUNNY_STORAGE_HOST.
+verify needs none of those; its base comes from --base or NWSYNC_PULL_BASE.
 `)
 }
 
@@ -76,12 +96,13 @@ func parseArgs(fs *flag.FlagSet, args []string) ([]string, error) {
 	}
 }
 
-func runEmit(args []string, stdout, stderr io.Writer) int {
+func runEmit(args []string, stderr io.Writer) int {
 	fs := flag.NewFlagSet("emit", flag.ContinueOnError)
 	fs.SetOutput(stderr)
 	as := fs.String("as", "", "published name of the artifact, when it differs from the key")
 	out := fs.String("out", "", "write to a local repository tree instead of uploading")
 	jobs := fs.Int("jobs", defaultEmitJobs, "resources to hash, compress and store at once")
+	verify := fs.Bool("verify", false, "read back and hash blobs that already exist instead of trusting their presence")
 	positional, err := parseArgs(fs, args)
 	if err != nil {
 		return exitUsage
@@ -101,17 +122,52 @@ func runEmit(args []string, stdout, stderr io.Writer) int {
 		As:           *as,
 		OutDir:       *out,
 		Jobs:         *jobs,
+		Verify:       *verify,
 	})
 	if err != nil {
 		fmt.Fprintf(stderr, "nwsync emit: %v\n", err)
 		return exitInternal
 	}
-	fmt.Fprintf(stdout, "emitted %s: %d resources, %d new blobs, index %s\n",
+	fmt.Fprintf(stderr, "emitted %s: %d resources, %d new blobs, index %s\n",
 		result.Name, result.Entries, result.BlobsWritten, result.ManifestPath)
 	return exitOK
 }
 
-func runAssemble(args []string, stdout, stderr io.Writer) int {
+func runVerify(args []string, stdout, stderr io.Writer, getenv func(string) string) int {
+	fs := flag.NewFlagSet("verify", flag.ContinueOnError)
+	fs.SetOutput(stderr)
+	base := fs.String("base", getenv("NWSYNC_PULL_BASE"), "pull zone base URL to read through")
+	sample := fs.Int("sample", 0, "check this many random blobs instead of all of them")
+	jobs := fs.Int("jobs", defaultEmitJobs, "blobs to fetch and hash at once")
+	positional, err := parseArgs(fs, args)
+	if err != nil {
+		return exitUsage
+	}
+	if len(positional) != 1 {
+		fmt.Fprintf(stderr, "nwsync verify: exactly one <manifest-sha1> is required\n")
+		return exitUsage
+	}
+
+	result, err := Verify(VerifyOptions{
+		ManifestSHA1: positional[0],
+		Base:         *base,
+		Sample:       *sample,
+		Jobs:         *jobs,
+		Log:          stderr,
+	})
+	if err != nil {
+		fmt.Fprintf(stderr, "nwsync verify: %v\n", err)
+		return exitInternal
+	}
+	fmt.Fprintf(stdout, "verified %d of %d blobs behind %d resources: %d failures, %d bytes checked\n",
+		result.Checked, result.Blobs, result.Entries, result.Failures, result.Bytes)
+	if result.Failures > 0 {
+		return exitDrift
+	}
+	return exitOK
+}
+
+func runAssemble(args []string, stderr io.Writer) int {
 	fs := flag.NewFlagSet("assemble", flag.ContinueOnError)
 	fs.SetOutput(stderr)
 	tlkKey := fs.String("tlk-key", "", "depot key of the TLK, which shadows nothing and merges last")
@@ -140,7 +196,7 @@ func runAssemble(args []string, stdout, stderr io.Writer) int {
 		fmt.Fprintf(stderr, "nwsync assemble: %v\n", err)
 		return exitInternal
 	}
-	fmt.Fprintf(stdout, "assembled manifest %s: %d resources, %s\n",
+	fmt.Fprintf(stderr, "assembled manifest %s: %d resources, %s\n",
 		result.SHA1, result.Entries, result.ManifestPath)
 	return exitOK
 }

internal/nwsync/sink.go

@@ -20,11 +20,17 @@ import (
 // upstream's output and ours can be diffed on a developer machine.
 type sink interface {
 	// putBlob stores one NWCompressedBuffer blob under its sha1 name and
-	// returns the bytes stored, or 0 if the blob was already there. Blob names
+	// returns the bytes stored, or 0 if a good copy was already there. Blob
-	// are content hashes, so an existing name is existing content — which is
+	// names are content hashes, so an existing name is normally taken as
-	// why body is a thunk: compression is the expensive part of emit and a
+	// existing content — which is why body is a thunk: compression is the
-	// blob that is already stored must not pay for it.
+	// expensive part of emit and a blob that is already stored must not pay
-	putBlob(sha1Hex string, body func() []byte) (int64, error)
+	// for it.
+	//
+	// verify stops trusting presence: the stored copy is read back, unwrapped
+	// and hashed, and replaced when it is not what its name claims. Without it
+	// an object written truncated, or written by an emitter since found broken,
+	// is skipped by every later emit forever and no backfill can repair it.
+	putBlob(sha1Hex string, verify bool, body func() []byte) (int64, error)
 	// putIndex stores a NSYM manifest and its sidecar under key, which is
 	// either an artifact-derived object key or a local path.
 	putIndex(key string, manifest, sidecar []byte) error
@@ -37,9 +43,15 @@ type sink interface {
 // dirSink writes a local NWSync repository tree.
 type dirSink struct{ root string }
 
-func (s dirSink) putBlob(sha1Hex string, body func() []byte) (int64, error) {
+func (s dirSink) putBlob(sha1Hex string, verify bool, body func() []byte) (int64, error) {
 	blob := blobPath(s.root, sha1Hex)
-	if _, err := os.Stat(blob); err == nil {
+	if !verify {
+		// Stat, not read: the common path must not pay to open every blob that
+		// is already there.
+		if _, err := os.Stat(blob); err == nil {
+			return 0, nil
+		}
+	} else if stored, err := os.ReadFile(blob); err == nil && blobMatchesName(stored, sha1Hex) == nil {
 		return 0, nil
 	}
 	if err := os.MkdirAll(filepath.Dir(blob), 0o755); err != nil {
@@ -91,8 +103,8 @@ type zoneSink struct {
 	zone  string
 }
 
-func (s zoneSink) putBlob(sha1Hex string, body func() []byte) (int64, error) {
+func (s zoneSink) putBlob(sha1Hex string, verify bool, body func() []byte) (int64, error) {
-	key := path.Join("data", "sha1", sha1Hex[0:2], sha1Hex[2:4], sha1Hex)
+	key := blobKey(sha1Hex)
 	// A throttled probe must never be read as "missing, re-upload" or as
 	// "present, skip", so only a confirmed Present skips the upload.
 	state, _, err := s.store.ProbeKey(s.ctx, key)
@@ -100,7 +112,24 @@ func (s zoneSink) putBlob(sha1Hex string, body func() []byte) (int64, error) {
 		return 0, fmt.Errorf("probe blob %s: %w", sha1Hex, err)
 	}
 	if state == depot.Present {
-		return 0, nil
+		if !verify {
+			return 0, nil
+		}
+		// The probe only proved the object exists. Read it back and hold it to
+		// its own name.
+		//
+		// This reads the storage API rather than the pull zone: emit holds the
+		// write credential, and a repair decision has to be made against the
+		// copy it is about to overwrite, not against an edge cache of it. A
+		// read that fails outright is a fault, not a verdict — treating it as
+		// "bad, re-upload" would turn a throttled zone into a full backfill.
+		stored, err := s.store.GetKey(s.ctx, key)
+		if err != nil {
+			return 0, fmt.Errorf("read back blob %s: %w", sha1Hex, err)
+		}
+		if blobMatchesName(stored, sha1Hex) == nil {
+			return 0, nil
+		}
 	}
 	data := body()
 	if err := s.put(key, data); err != nil {

internal/nwsync/verify.go

@@ -0,0 +1,233 @@
+package nwsync
+
+import (
+	"crypto/sha1"
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"io"
+	"math/rand/v2"
+	"net/http"
+	"path"
+	"sort"
+	"strconv"
+	"sync"
+	"time"
+
+	"git.westgate.pw/ShadowsOverWestgate/sow-tools/internal/erf"
+)
+
+// defaultPullBase is the public NWSync host, which is a Bunny pull zone fronting
+// the storage zone. Verify reads through it rather than through the storage API
+// on purpose: what matters is the bytes a client is served, edge behaviour
+// included, not what the origin believes it holds.
+const defaultPullBase = "https://nwsync.westgate.pw"
+
+// errBlobMissing marks an object the zone does not serve at all, as distinct
+// from one it serves badly.
+var errBlobMissing = errors.New("missing")
+
+// blobSource reads one object out of the zone by key. Verify never writes and
+// never authenticates, so this is deliberately narrower than sink.
+type blobSource interface {
+	get(key string) ([]byte, error)
+	describe(key string) string
+}
+
+// pullZone reads the zone over plain HTTP, with no credential.
+type pullZone struct {
+	base   string
+	client *http.Client
+}
+
+func newPullZone(base string) blobSource {
+	if base == "" {
+		base = defaultPullBase
+	}
+	return pullZone{
+		base: base,
+		// A full sweep is tens of thousands of small requests, so connections
+		// have to be reused; the default transport does that already.
+		client: &http.Client{Timeout: 60 * time.Second},
+	}
+}
+
+func (z pullZone) describe(key string) string { return z.base + "/" + key }
+
+func (z pullZone) get(key string) ([]byte, error) {
+	resp, err := z.client.Get(z.describe(key))
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode == http.StatusNotFound || resp.StatusCode == http.StatusGone {
+		_, _ = io.Copy(io.Discard, resp.Body)
+		return nil, errBlobMissing
+	}
+	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+		_, _ = io.Copy(io.Discard, resp.Body)
+		return nil, fmt.Errorf("unexpected status %d", resp.StatusCode)
+	}
+	return io.ReadAll(resp.Body)
+}
+
+// VerifyOptions describes one verify run.
+type VerifyOptions struct {
+	ManifestSHA1 string     // the merged manifest to verify
+	Base         string     // pull zone base URL; empty means defaultPullBase
+	Sample       int        // check this many random blobs; 0 means all of them
+	Jobs         int        // blobs in flight at once; 0 means defaultEmitJobs
+	Source       blobSource // test seam; nil means the pull zone at Base
+	Log          io.Writer  // per-blob failures land here; nil discards them
+}
+
+// VerifyResult reports what one verify run found.
+type VerifyResult struct {
+	Entries  int   // resources the manifest names
+	Blobs    int   // distinct blobs behind those resources
+	Checked  int   // blobs actually fetched
+	Failures int   // blobs that failed a check
+	Bytes    int64 // uncompressed bytes verified
+}
+
+// Verify reads a published manifest and its blobs the way a client reads them,
+// and reports every blob that is not what the manifest says it is.
+//
+// Presence is not correctness. emit skips an object that already exists on the
+// strength of a 1-byte range GET, so a truncated or wrongly framed object is
+// skipped by every later emit forever and the backfill cannot repair it. This is
+// the only thing upstream of a player's client that can tell that has happened.
+//
+// Every blob is decompressed and hashed. A Content-Length check would pass the
+// exact failure mode being hunted — a byte-correct-looking object whose contents
+// are wrong — and a round-trip check alone would pass a frame that omits its
+// content size, because Go's decoder is more capable than the client's (#86).
+func Verify(options VerifyOptions) (VerifyResult, error) {
+	// The argument is interpolated straight into a URL path, so it is checked
+	// rather than trusted: exactly 20 bytes of hex, nothing else.
+	if sum, err := hex.DecodeString(options.ManifestSHA1); err != nil || len(sum) != sha1.Size {
+		return VerifyResult{}, fmt.Errorf("%q is not a manifest sha1", options.ManifestSHA1)
+	}
+	source := options.Source
+	if source == nil {
+		source = newPullZone(options.Base)
+	}
+	log := options.Log
+	if log == nil {
+		log = io.Discard
+	}
+
+	manifestKey := path.Join("manifests", options.ManifestSHA1)
+	data, err := source.get(manifestKey)
+	if err != nil {
+		return VerifyResult{}, fmt.Errorf("%s: %w", source.describe(manifestKey), err)
+	}
+	// A manifest is named after its own sha1, so this catches the zone serving
+	// a different manifest — or a truncated one — before any blob is fetched.
+	if got := hex.EncodeToString(sha1Sum(data)); got != options.ManifestSHA1 {
+		return VerifyResult{}, fmt.Errorf("%s hashes to %s, not the manifest asked for",
+			source.describe(manifestKey), got)
+	}
+	entries, err := readManifest(data)
+	if err != nil {
+		return VerifyResult{}, fmt.Errorf("%s: %w", source.describe(manifestKey), err)
+	}
+
+	// A manifest names one blob many times over: mappings share a sha1, and so
+	// do resrefs with identical contents. Fetch each blob once.
+	blobs := make([]Entry, 0, len(entries))
+	seen := make(map[[20]byte]bool, len(entries))
+	for _, entry := range entries {
+		if seen[entry.SHA1] {
+			continue
+		}
+		seen[entry.SHA1] = true
+		blobs = append(blobs, entry)
+	}
+
+	result := VerifyResult{Entries: len(entries), Blobs: len(blobs)}
+	checking := blobs
+	if options.Sample > 0 && options.Sample < len(blobs) {
+		// A full sweep of the live manifest is ~69,000 objects and ~15 GB, so
+		// sampling is what makes verifying a routine act rather than an event.
+		picks := rand.Perm(len(blobs))[:options.Sample]
+		checking = make([]Entry, 0, options.Sample)
+		for _, i := range picks {
+			checking = append(checking, blobs[i])
+		}
+	}
+	result.Checked = len(checking)
+
+	jobs := options.Jobs
+	if jobs < 1 {
+		jobs = defaultEmitJobs
+	}
+	var (
+		mu       sync.Mutex
+		failures []string
+	)
+	work := make(chan Entry)
+	var wg sync.WaitGroup
+	for range jobs {
+		wg.Go(func() {
+			for entry := range work {
+				fault := checkEntry(source, entry)
+				mu.Lock()
+				if fault != "" {
+					failures = append(failures, fault)
+				} else {
+					result.Bytes += int64(entry.Size)
+				}
+				mu.Unlock()
+			}
+		})
+	}
+	for _, entry := range checking {
+		work <- entry
+	}
+	close(work)
+	wg.Wait()
+
+	// Workers finish in any order; a report an operator can diff must not.
+	sort.Strings(failures)
+	for _, fault := range failures {
+		fmt.Fprintln(log, fault)
+	}
+	result.Failures = len(failures)
+	return result, nil
+}
+
+// checkEntry fetches one blob and returns a one-line fault, or "" if it is
+// exactly what the manifest entry says it is.
+func checkEntry(source blobSource, entry Entry) string {
+	// Name the resource, not just the hash: an operator has to find the thing
+	// in a hak, and a bare sha1 says nothing about where to look.
+	extension, ok := erf.ExtensionForResourceType(entry.ResType)
+	if !ok {
+		extension = strconv.Itoa(int(entry.ResType))
+	}
+	where := fmt.Sprintf("%s (%s.%s)", entry.sha1Hex(), entry.ResRef, extension)
+	blob, err := source.get(blobKey(entry.sha1Hex()))
+	if err != nil {
+		if errors.Is(err, errBlobMissing) {
+			return where + ": missing"
+		}
+		return where + ": unreadable: " + err.Error()
+	}
+	data, err := inspectBlob(blob)
+	if err != nil {
+		return where + ": " + err.Error()
+	}
+	if uint32(len(data)) != entry.Size {
+		return fmt.Sprintf("%s: size mismatch: %d bytes, manifest says %d", where, len(data), entry.Size)
+	}
+	if sha1.Sum(data) != entry.SHA1 {
+		return fmt.Sprintf("%s: hash mismatch: contents hash to %x", where, sha1.Sum(data))
+	}
+	return ""
+}
+
+func sha1Sum(data []byte) []byte {
+	sum := sha1.Sum(data)
+	return sum[:]
+}

internal/nwsync/verify_test.go

@@ -0,0 +1,285 @@
+package nwsync
+
+import (
+	"bytes"
+	"crypto/sha1"
+	"encoding/hex"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"github.com/klauspost/compress/zstd"
+)
+
+// verifyFixture emits two haks and a TLK into a fake zone, assembles them, and
+// hands back a verifier reading that zone the way a client would.
+type verifyFixture struct {
+	*zoneSinkFixture
+	manifestSHA1 string
+}
+
+func newVerifyFixture(t *testing.T) *verifyFixture {
+	t.Helper()
+	zone := newZoneFixture(t)
+	dir := t.TempDir()
+	hak := filepath.Join(dir, "sow_test_01.hak")
+	// A payload under 256 bytes is the one the frame-header check exists for.
+	writeHak(t, hak, map[string][]byte{
+		"bloodstain1.tga": []byte("small"),
+		"appearance.2da":  bytes.Repeat([]byte("2DA V2.0\n"), 200),
+	})
+	tlk := filepath.Join(dir, "sow_tlk.tlk")
+	if err := os.WriteFile(tlk, []byte("TLK V3.0 payload"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	zone.emit(t, hak)
+	if _, err := Emit(EmitOptions{
+		ArtifactKey: artifactKey(t, tlk), ArtifactPath: tlk, As: "sow_tlk.tlk", Sink: zone.sink,
+	}); err != nil {
+		t.Fatalf("emit tlk: %v", err)
+	}
+	assembled, err := Assemble(AssembleOptions{
+		ArtifactKeys: []string{artifactKey(t, hak)}, TLKKey: artifactKey(t, tlk), Sink: zone.sink,
+	})
+	if err != nil {
+		t.Fatalf("assemble: %v", err)
+	}
+	return &verifyFixture{zoneSinkFixture: zone, manifestSHA1: assembled.SHA1}
+}
+
+func (f *verifyFixture) verify(t *testing.T, sample int) (VerifyResult, string, error) {
+	t.Helper()
+	var log bytes.Buffer
+	result, err := Verify(VerifyOptions{
+		ManifestSHA1: f.manifestSHA1,
+		Sample:       sample,
+		Source:       f.zone.pullZone(),
+		Log:          &log,
+	})
+	return result, log.String(), err
+}
+
+// keyOf is where a resource's blob lives, addressed by the sha1 of its
+// uncompressed bytes — the same path the client requests.
+func keyOf(body []byte) string {
+	sum := sha1.Sum(body)
+	return blobKey(hex.EncodeToString(sum[:]))
+}
+
+func TestVerifyPassesACleanZone(t *testing.T) {
+	fixture := newVerifyFixture(t)
+	result, log, err := fixture.verify(t, 0)
+	if err != nil {
+		t.Fatalf("verify: %v", err)
+	}
+	if result.Failures != 0 {
+		t.Errorf("verify reported %d failures on a clean zone: %s", result.Failures, log)
+	}
+	// A default run is a full sweep, so it must reach every blob the manifest
+	// names — not some of them.
+	if result.Checked != result.Blobs || result.Blobs == 0 {
+		t.Errorf("checked %d of %d blobs; a full sweep must check all of them", result.Checked, result.Blobs)
+	}
+}
+
+func TestVerifyReportsAMissingBlob(t *testing.T) {
+	fixture := newVerifyFixture(t)
+	key := keyOf([]byte("small"))
+	fixture.zone.mu.Lock()
+	delete(fixture.zone.objects, key)
+	fixture.zone.mu.Unlock()
+
+	result, log, err := fixture.verify(t, 0)
+	if err != nil {
+		t.Fatalf("verify: %v", err)
+	}
+	if result.Failures != 1 {
+		t.Fatalf("reported %d failures, want 1: %s", result.Failures, log)
+	}
+	if !strings.Contains(log, "missing") {
+		t.Errorf("a deleted blob was not reported as missing: %s", log)
+	}
+}
+
+func TestVerifyReportsATruncatedBlob(t *testing.T) {
+	fixture := newVerifyFixture(t)
+	key := keyOf([]byte("small"))
+	fixture.zone.mu.Lock()
+	fixture.zone.objects[key] = fixture.zone.objects[key][:blobHeaderBytes+4]
+	fixture.zone.mu.Unlock()
+
+	result, log, err := fixture.verify(t, 0)
+	if err != nil {
+		t.Fatalf("verify: %v", err)
+	}
+	if result.Failures != 1 {
+		t.Fatalf("reported %d failures, want 1: %s", result.Failures, log)
+	}
+	if !strings.Contains(log, "framing") {
+		t.Errorf("a truncated blob was not reported as malformed framing: %s", log)
+	}
+}
+
+// TestVerifyRejectsABlobWithNoDeclaredFrameContentSize is the check that #86
+// slipped past: the blob decompresses to exactly the right bytes, so a verifier
+// that only round-trips certifies it, yet the client cannot decode it.
+func TestVerifyRejectsABlobWithNoDeclaredFrameContentSize(t *testing.T) {
+	fixture := newVerifyFixture(t)
+	body := []byte("small")
+	key := keyOf(body)
+
+	fixture.zone.mu.Lock()
+	good := fixture.zone.objects[key]
+	encoder, err := zstd.NewWriter(nil, zstd.WithEncoderConcurrency(1))
+	if err != nil {
+		t.Fatal(err)
+	}
+	bad := append(append([]byte{}, good[:blobHeaderBytes]...), encoder.EncodeAll(body, nil)...)
+	fixture.zone.objects[key] = bad
+	fixture.zone.mu.Unlock()
+
+	if frameDeclaresContentSize(bad[blobHeaderBytes:]) {
+		t.Fatal("the fixture blob declares a content size; it cannot exercise the check")
+	}
+	if got, err := decompressBlob(bad); err != nil || !bytes.Equal(got, body) {
+		t.Fatalf("the fixture blob must round trip, or it proves nothing: %v", err)
+	}
+
+	result, log, err := fixture.verify(t, 0)
+	if err != nil {
+		t.Fatalf("verify: %v", err)
+	}
+	if result.Failures != 1 {
+		t.Fatalf("reported %d failures, want 1: %s", result.Failures, log)
+	}
+	if !strings.Contains(log, "content size") {
+		t.Errorf("undeclared frame content size was not the reported reason: %s", log)
+	}
+}
+
+func TestVerifyReportsWrongContents(t *testing.T) {
+	fixture := newVerifyFixture(t)
+	key := keyOf([]byte("small"))
+	fixture.zone.mu.Lock()
+	// Valid framing, valid zstd, wrong bytes: only decompressing and hashing
+	// can see this, which is why Content-Length is not enough.
+	fixture.zone.objects[key] = compressBlob([]byte("wrong"))
+	fixture.zone.mu.Unlock()
+
+	result, log, err := fixture.verify(t, 0)
+	if err != nil {
+		t.Fatalf("verify: %v", err)
+	}
+	if result.Failures != 1 {
+		t.Fatalf("reported %d failures, want 1: %s", result.Failures, log)
+	}
+	if !strings.Contains(log, "hash mismatch") {
+		t.Errorf("wrong contents were not reported as a hash mismatch: %s", log)
+	}
+}
+
+func TestVerifyReportsAShortBlob(t *testing.T) {
+	fixture := newVerifyFixture(t)
+	key := keyOf([]byte("small"))
+	fixture.zone.mu.Lock()
+	// Well-formed all the way down and simply too short — the shape a killed
+	// upload leaves behind, and the one a Content-Length check would pass.
+	fixture.zone.objects[key] = compressBlob([]byte("sma"))
+	fixture.zone.mu.Unlock()
+
+	result, log, err := fixture.verify(t, 0)
+	if err != nil {
+		t.Fatalf("verify: %v", err)
+	}
+	if result.Failures != 1 {
+		t.Fatalf("reported %d failures, want 1: %s", result.Failures, log)
+	}
+	if !strings.Contains(log, "size mismatch") {
+		t.Errorf("a short blob was not reported as a size mismatch: %s", log)
+	}
+}
+
+func TestVerifyFailsWhenTheManifestIsNotTheOneAsked(t *testing.T) {
+	fixture := newVerifyFixture(t)
+	fixture.zone.mu.Lock()
+	fixture.zone.objects["manifests/"+fixture.manifestSHA1] = []byte("NSYM garbage")
+	fixture.zone.mu.Unlock()
+
+	if _, _, err := fixture.verify(t, 0); err == nil {
+		t.Fatal("verify accepted a manifest that is not the one requested")
+	}
+}
+
+func TestVerifySampleChecksFewerBlobs(t *testing.T) {
+	fixture := newVerifyFixture(t)
+	result, log, err := fixture.verify(t, 1)
+	if err != nil {
+		t.Fatalf("verify: %v", err)
+	}
+	if result.Checked != 1 {
+		t.Errorf("--sample 1 checked %d blobs, want 1: %s", result.Checked, log)
+	}
+	if result.Blobs <= result.Checked {
+		t.Errorf("sampling %d of %d blobs is not a sample", result.Checked, result.Blobs)
+	}
+}
+
+// TestEmitVerifyReplacesABlobThatIsNotItsName covers the reason #86 could not be
+// fixed by the encoder alone: emit skips whatever is already present, so every
+// blob published by the broken encoder stays broken until emit stops trusting
+// presence.
+func TestEmitVerifyReplacesABlobThatIsNotItsName(t *testing.T) {
+	fixture := newZoneFixture(t)
+	dir := t.TempDir()
+	hak := filepath.Join(dir, "sow_test_01.hak")
+	body := []byte("blood")
+	writeHak(t, hak, map[string][]byte{"bloodstain1.tga": body})
+
+	emit := func(verify bool) EmitResult {
+		t.Helper()
+		result, err := Emit(EmitOptions{
+			ArtifactKey:  artifactKey(t, hak),
+			ArtifactPath: hak,
+			Sink:         fixture.sink,
+			Verify:       verify,
+		})
+		if err != nil {
+			t.Fatalf("emit (verify=%v): %v", verify, err)
+		}
+		return result
+	}
+
+	emit(false)
+	key := keyOf(body)
+	encoder, err := zstd.NewWriter(nil, zstd.WithEncoderConcurrency(1))
+	if err != nil {
+		t.Fatal(err)
+	}
+	fixture.zone.mu.Lock()
+	good := fixture.zone.objects[key]
+	fixture.zone.objects[key] = append(append([]byte{}, good[:blobHeaderBytes]...), encoder.EncodeAll(body, nil)...)
+	fixture.zone.mu.Unlock()
+
+	if plain := emit(false); plain.BlobsWritten != 0 {
+		t.Fatalf("a plain re-emit wrote %d blobs; it is supposed to trust presence", plain.BlobsWritten)
+	}
+	if verified := emit(true); verified.BlobsWritten != 1 {
+		t.Fatalf("--verify wrote %d blobs, want 1 (the bad copy must be replaced)", verified.BlobsWritten)
+	}
+
+	fixture.zone.mu.Lock()
+	repaired := fixture.zone.objects[key]
+	fixture.zone.mu.Unlock()
+	if !bytes.Equal(repaired, good) {
+		t.Error("the replaced blob is not what the current encoder produces")
+	}
+	if _, err := inspectBlob(repaired); err != nil {
+		t.Errorf("the replaced blob still fails inspection: %v", err)
+	}
+
+	// A second verifying run has nothing left to repair.
+	if again := emit(true); again.BlobsWritten != 0 {
+		t.Errorf("--verify rewrote %d good blobs", again.BlobsWritten)
+	}
+}

internal/nwsync/zone_test.go

@@ -21,6 +21,13 @@ type fakeZone struct {
 	objects map[string][]byte
 	puts    []string
 	failOn  func(key string) bool // when true, the PUT fails
+	url     string                // base the same objects are readable at
+}
+
+// pullZone reads the fake zone the way the public pull zone is read: plain
+// unauthenticated GETs, no storage API.
+func (z *fakeZone) pullZone() blobSource {
+	return newPullZone(z.url)
 }
 
 func newFakeZone(t *testing.T) (*fakeZone, func(string) string) {
@@ -28,6 +35,7 @@ func newFakeZone(t *testing.T) (*fakeZone, func(string) string) {
 	zone := &fakeZone{objects: map[string][]byte{}}
 	server := httptest.NewServer(zone)
 	t.Cleanup(server.Close)
+	zone.url = server.URL + "/sow-nwsync"
 	getenv := func(name string) string {
 		switch name {
 		case "NWSYNC_STORAGE_ZONE":